Hadrian's Wall, established from the 2nd century AD as the frontier of Roman rule in the British Isles, enables England and Wales (as they will later become) to settle down together as Britannia, the most northerly Roman province.

On the whole the Celtic chieftains of Britain adapt willingly to Roman customs and comforts. They learn to live in villas, they speak Latin, they benefit from trading links with the empire (British wheat and wool are much in demand), and they become Roman citizens. The tribal centres develop into thriving Roman towns, around the forum (market place) and basilica (town hall).

Towns of this kind, serving as the capitals of British tribal rulers enjoying Roman support, include Winchester, Dorchester, Cirencester and Canterbury. London develops at the same period, but as a centre of trade at the focal point of the network of Roman roads. Bath, with its hot springs, becomes Britain's first resort.

Different in kind are the essentially Roman headquarters of Chester, Caerleon and York (where Constantine is proclaimed emperor in 306). These are the permanent bases of the Roman legions in Britain. Other modern cities, including Lincoln, Colchester and St Albans, derive from Roman municipalities - founded for new settlers, such as men retiring from the legions.

Roman Britain never achieves the prosperity or sophistication of Roman Gaul, and it has the disadvantage of being cut off from the centre whenever Gaul is controlled by rebellious Roman armies or invading barbarians. Even so, Britain has much in common with other provinces of the empire. It has its great villas (a palace at Fishbourne, discovered in 1960, is one of the grandest, with superb mosaic floors). And it has its choice of the empire's rival religions.

By the late 3rd century Mithras and Jesus Christ compete for attention. In 314 the winning side, the Christians, are sufficiently well organized to send three bishops from Britain to a council in Gaul.

Britannia in decline: 5th - 6th century AD

The decline of Roman Britain is like the withering of a limb at the extremity of an ailing body. In unsettled times, in the late 4th century, western emperors withdraw legions from Britain for their own local purposes. Once Gaul is in the hands of barbarian rulers in the 5th century, blocking the route from Rome, no new replacements arrive.

The Roman British find themselves extremely vulnerable. They have defences in the north, but none in the southeast - the direction of Rome, and supposedly secure. It is from this undefended side that danger comes. German tribes moving south and west into Gaul have Britain in their sights.

The main threat is from two tribal groups pressing southwest from the Baltic coast. They are the Angles and the Saxons. The subsequent Anglo-Saxon basis of England, and of the English language, speaks for their success.

The Romanized Celts, deprived of their Roman legions, prove unable to resist these more primitive and ferocious intruders - though their struggle is personified in a legendary hero, King Arthur. By the 6th century the Celtic chieftains are confined to mountainous Wales. The fertile plains of England are occupied now by Angles, Saxons and other German tribes from roughly the same area, such as Jutes and Frisians. Their chieftains set about establishing themselves as regional kings.